The global positioning system (GPS) is based on an earth-orbiting constellation of twenty-four satellite vehicles each broadcasting its precise location and ranging information. From any location on or near the earth, a GPS receiver with an unobstructed view of the sky should be able to track at least four satellite vehicles thereby being able to calculate the receiver's precise latitude, longitude, and elevation. Each satellite vehicle constantly transmits two signals, generally referred to as L1 and L2. The L1 signal from a satellite vehicle contains a unique pseudo-random noise code ranging signal with a chipping frequency of 1.023 MHz, system data with a bitrate frequency of 50 Hz, and an encrypted precise-code (y-code) with a chipping frequency of 10.23 MHz all being modulated onto a carrier frequency of 1575.42 MHz. The L2 signal consists of the system data and y-code being modulated onto a carrier frequency of 1227.60 MHz. In order to calculate a three-dimensional location, a receiver must determine the distance from itself to at least four satellite vehicles. This is accomplished by first determining the location of at least four satellite vehicles using ephemeris data received from the satellites. Once the locations of the satellites have been determined, the distance from the receiver to each of the satellites is calculated based upon an estimate of the receiver's position. The measurement of the distance from the receiver to a satellite is based on the amount of time that elapsed between the transmission of a ranging signal from each satellite vehicle and the reception of that chip symbol by the receiver. In particular, the estimated position of the receiver is then corrected based upon a time epoch associated with the received ranging signal.
The accuracy of a GPS receiver depends on the accuracy with which the receiver is capable of measuring the time that has elapsed between the broadcast of the range information by a satellite vehicle and the reception of the information by the receiver. There are several factors that reduce the accuracy of the time measurement in the receiver design, including the sampling bandwidth of the receiver, the number of sampling bits, errors caused by internally generated noise, and external interference. Additional system factors that cause reduction of accuracy include errors in the ephemeris data (location of the satellite), errors caused by delays due to the ionosphere and troposphere, and multipath errors caused by reflected signals entering the receiver antenna.
In order to obtain the precise location of the receiver, a received GPS signal must be converted from an RF signal to a baseband signal before the GPS information can be extracted from the received GPS signal. In the present state of the art, a received GPS signal is downconverted to an IF signal, and the IF signal is converted to baseband using filtering circuitry such as SAW filters, which are not compatible for integration onto a semiconductor die with the other components of the GPS receiver.